Conventionally, there is a communication system which provides a high quality to a user by managing a band. For example, in case of a wireless communication system using a standard typified by the IEEE802.11e standard, there is a method in which a master unit device performs a scheduling for allocating a communication time band of a slave unit based on a band amount (time or a data rate, for example) requested by the slave unit, thereby granting a transmission right to the slave unit (patent document 1, for example). FIG. 17 is a diagram illustrating a conventional band management method disclosed in patent document 1.
In patent document 1, a master unit device regularly transmits a beacon, which is a control frame, to a slave unit, and grants a transmission right to the slave unit within a time period divided by the beacon (hereinafter referred to as beacon segment). Thus, the master unit device manages a communication time band of the slave unit. Specifically, the master unit device transmits, to the slave unit, a frame indicating a grant of a transmission right (CF-Poll), thereby granting, to the slave unit, a TXOP (Transmission Opportunity) which is an occupancy period of a transmission path. The slave unit which has acquired the TXOP from the CF-Poll is allowed to perform frame transmission, occupying a transmission path during the occupancy period. As such, each specific slave unit occupies a transmission path at predetermined time intervals, thereby avoiding frame collision with other slave units. Thus, it becomes possible to provide a high communication quality to the user.
On the other hand, along with such improved communication quality, a copyright technique has been developed for preventing a content, such as video or the like, from being distributed in a high-quality network in a random manner. For example, DTCP-IP (Digital Transmission Content Protection over Internet Protocol) is known as an algorithm which realizes copyright protection in a network. DTCP-IP has a mechanism in which an authentication/key exchange is performed between Source (video transmission side) and Sink (video reception side) on a content such as video data which requires the copyright protection, thereby encrypting the video data so as to be transferred (see non-patent document 1 for details). FIG. 18 shows authentication steps in DTCP-IP. When performing the DTCP-IP authentication, an RTT (Round Trip Time) test is implemented for limiting a distribution area. The RTT test is a test measuring a time required from when Source transmits a frame (RTT_TEST(MAC1A).CMD) to Sink to when a response frame (ACCEPTED(MAC2B).RSP) transmitted from Sink arrives at Source, i.e., a time required for a signal to travel between Source and Sink. The RTT test has a time limit equal to or less than 7 msec, and when a required time satisfies the time limit within 1023 times of test trials, that is, within 1022 times of test retrials, the authentication steps are completed, thereby allowing Source to distribute content data to Sink. On the other hand, all results from 1023 times of the test trials indicate a time period beyond the time limit, it is determined that Source and Sink exist in different buildings, and thus the content data is not to be distributed from Source to Sink.
With the improved communication quality provided by managed communication time band and the copyright protection technology using DTCP-IP, a high-quality video transmission is being realized in a network established in a house.
Patent document 1: Japanese Laid-Open Patent Publication No. 2005-143118 (pages 3-4, FIG. 1)
Non-patent document: DTCP Volume 1 Supplement E Revision1.2 (Informational Version) (pages 24-27, FIG. 4)